1. Field of the Invention
The present invention relates to an apparatus for a high efficiency power transmitter in a wireless communication system. More particularly the present invention relates to an apparatus for enhancing performance of an envelope elimination and restoration (EER) power transmitter in a wireless communication system.
2. Description of the Related Art
Recently, with the development of mobile communication technologies, various multi-media services in a wireless environment are being provided. To provide these multi-media services, a wireless communication system requires a high efficiency power transmitter. A high efficiency power transmitter is required for processing the large amount of data that is generated by the multi-media services.
Two important characteristics of a power transmitter in a wireless communication system are linearity and efficiency. However, it is difficult to satisfy both of these characteristics of a general power transmitter simultaneously. That is, because of a tradeoff relationship between the linearity and the efficiency of a power transmitter, satisfying one characteristic leads to a poor result regarding the other characteristic.
A wireless communication system can satisfy the above two characteristics of the power transmitter by using an envelope elimination and restoration (EER) power transmitter. As shown in FIG. 1, a conventional EER power transmitter divides a transmission signal into a size component and a phase component. The EER amplifies a size of a radio frequency (RF) signal for the phase component, thereby reducing distortion of an amplifier itself and enabling high efficiency amplification. In this case, the EER power transmitter uses the size component of the transmission signal as an output bias voltage of an amplifier which amplifies a size of the RF signal for the phase component. Therefore, it can restore the amplified transmission signal just by amplifying the RF signal for the phase component.
FIG. 1 is a diagram illustrating a conventional EER power transmitter.
As shown in FIG. 1, a conventional EER power transmitter comprises a signal divider 100, a size amplifier 110, an orthogonal modulator 120, a local oscillator 130 and a high efficiency amplifier 140.
The signal divider 100 divides a transmission signal of the EER power transmitter into a size component and a phase component. The signal divider 100 transmits the size component to the size amplifier 110 and outputs the phase component to the orthogonal modulator 120.
The size amplifier 110 amplifies the size component supplied from the signal divider 100 and outputs the amplified size component to the high efficiency amplifier 140.
The orthogonal modulator 120 modulates and outputs the phase component supplied from the signal divider 100 into an RF signal according to an RF carrier supplied from the local oscillator 130.
The local oscillator 130 generates the RF carrier for modulating the phase component into the RF signal and provides the RF carrier to the orthogonal modulator 120.
The high efficiency amplifier 140 amplifies a size of the RF signal supplied from the orthogonal modulator 120 by using a size signal supplied from the size amplifier 110 as an output bias voltage. In this case, since the high efficiency amplifier 140 amplifies the RF signal of the phase component supplied from the orthogonal modulator 120, it can amplify the RF signal without distortion thereof.
As described above, since the amplifier of a conventional EER power transmitter in a wireless communication system amplifies the phase component by using the size component as an output bias voltage of the amplifier, it can satisfy both efficiency and linearity of the power transmitter simultaneously.
Accordingly, a performance of a conventional EER power transmitter is determined by the high efficiency amplifier and a bias modulator which generates a bias voltage of the high efficiency amplifier.
That is, the linearity and efficiency of the bias modulator, i.e., the size amplifier, directly affects the linearity and efficiency of a conventional EER power transmitter. For instance, the size amplifier uses a high output voltage in order to make the high efficiency amplifier restore a peak to average ratio (PAR) and maintain linearity. In this case, the size amplifier has a problem in that efficiency decreases due to the high output voltage.
When the size component of a transmission signal is amplified in the size amplifier, the high efficiency amplifier has high impedance in a modulation frequency band of the output bias voltage supplied from the size amplifier. Hence, the high efficiency amplifier has a problem of a memory effect due to an impedance difference in the modulation frequency band.